1. Field of the Invention
The present invention relates to a photoelectric sensor, and more specifically relates to improvement in a photoelectric sensor that emits detection light and receives its reflected light, to thereby detect a workpiece.
2. Description of Related Art
There is conventionally known a photoelectric sensor that emits detection light to a predetermined detection position and receives its reflected light, to thereby detect presence or absence of a workpiece. As this kind of photoelectric sensors, there are known are a type which uses a difference in light receiving amount due to the presence or absence of the workpiece, and a type which uses triangulation. In the case of the type of the photoelectric sensor which uses triangulation, a distance to a reflection surface is measured by triangulation and the distance is compared with a threshold, to detect the presence or absence of the workpiece.
The distance to the reflection surface can also be obtained by measuring light transmission time. In other words, it is considered that, when time from emission of the detection light to reception of the reflected light can be measured, the distance to the reflection surface can be obtained and the workpiece can be detected without using triangulation.
However, when the workpiece is to be detected by measuring the light transmission time in the photoelectric sensor, the reflected light needs to be sampled in an extremely short period, which has been problematic. A speed of light is 3.0×108 m/s, and the light travels 30 mm in 100 ps. Therefore, for example, when a distance to the workpiece is to be detected with an accuracy of 15 mm, it is necessary to sample the reflected light by an order of 100 ps.
Sampling the reflected light at such a speed is not easy, and in order to achieve this, countermeasures for heat generation is required, which is considered to increase the size of the device as a whole.